1. Field of the Invention
The present invention relates to an image forming apparatus and a sheet processing apparatus that perform serial communication with one another.
2. Description of the Related Art
Conventionally, serial communication is normally used for communication between an image forming apparatus and a sheet processing apparatus (finisher). In this case, three-wire serial communication is used between the conventional image forming apparatus and the finisher, with a controller (ECONT) for controlling image forming operations by the image forming apparatus on the master side. The three-wire serial communication lines are composed of a serial communication line from the image forming apparatus to the finisher, a serial communication line from the finisher to the image forming apparatus, and a clock signal line for serial communication from the image forming apparatus to the finisher. When there are signals that cannot be handled by such three-wire serial communication, other communication lines are separately provided between the image forming apparatus and the finisher.
A finisher is usually designed as an option to be connected to an image forming apparatus, so that the image forming apparatus needs to determine whether a finisher has been provided. A configuration where the image forming apparatus is provided with separate signal lines for determining whether options have been connected is conventionally known, though this means that in addition to the three-wire serial communication lines described above, extra signal lines have to be provided for detecting options. Increasing the number of signal lines leads to increases in the cost of the connectors that connect the cables between the image forming apparatus and the finisher. Also, for ease of mounting, a number of connectors are used to connect the cables between the image forming apparatus and the finisher, so that increases in the cost per connector cannot be ignored for the apparatus as a whole. Furthermore, there is a tendency for the amount of noise, which is radiated from serial communication lines due to the antenna effect of the signal lines, to increase in keeping with the number of signal lines. It is also necessary to provide a CPU of the image forming apparatus with a port for inputting signals from the signal lines for detecting options, which suppresses the freedom with which the ports can be used for the CPU even though the CPU itself is often subject to restrictions regarding the number of ports.
A configuration is known where predetermined communication for device detection is performed between the image forming apparatus and optional devices so that the image forming apparatus can judge whether options are provided, though with this configuration the image forming apparatus cannot judge instantly whether options are provided. Conventionally, it is judged whether options have been provided every time the power of the image forming apparatus is switched on, though since the starting up of the image forming apparatus itself takes time, the communication time taken to judge whether options are provided has not been a problem compared to the time taken by the starting up of the image forming apparatus. However, in recent years, to reduce power consumption, image forming apparatuses have been designed so as to frequently switch to power saving mode and the startup time of image forming apparatuses has been reduced. Since it is necessary to detect whether options are connected whenever an image forming apparatus returns from power saving mode, if an image forming apparatus judges whether options are attached by communicating with the options in the conventional manner, it is not possible for the image forming apparatus to quickly start up from the power-saving mode.
The present invention has been devised in view of the above described problem, and it is an object of the present invention to provide a sheet processing apparatus and an image forming apparatus, which make it possible to determine whether the sheet processing apparatus is attached to the image forming apparatus in a short time and without increasing the number of signal lines.
To attain the above object, in a first aspect of the present invention, there is provided a sheet processing apparatus to be attached to an image forming apparatus, comprising a controller that controls the sheet processing apparatus, and a serial communication line that performs serial communication from the controller to the image forming apparatus, wherein the controller is responsive to supplying of power to the controller, for setting the serial communication line at a high level.
In a preferred form, the sheet processing apparatus according to the first aspect comprises a resistor, and a first switching element, and wherein the serial communication line is disposed such that a voltage is applied to the serial communication line via the resistance, the serial communication line is grounded via the first switching element, and the controller controls the first switching element.
Preferably, the controller sets the serial communication line at the high level by turning off the first switching element.
More preferably, the controller performs serial communication with the image forming apparatus by turning the first switching element on and off.
Preferably, the first switching element comprises a transistor.
Also preferably, the serial communication line is disposed relative to the image forming apparatus such that a voltage of a power supply supplied by the image forming apparatus is applied to the serial communication line via the resistance.
Further preferably, the sheet processing apparatus according to the first aspect comprises a second switching element, and wherein power supplied from the image forming apparatus is supplied to the controller and the serial communication line via the second switching element, and wherein the second switching element is controlled in accordance with a signal on a power supply control signal line from the image forming apparatus.
Advantageously, the second switching element comprises a field effect transistor (FET).
Also advantageously, the sheet processing apparatus according to the first aspect further comprises a second serial communication line that performs serial communication from the image forming apparatus to the controller.
Preferably, the image forming apparatus is responsive to supplying of the power to the controller, for determining whether the serial communication line is at the high level, and determines that the sheet processing apparatus is attached when the serial communication line is at the high level.
Also preferably, the image forming apparatus judges whether the serial communication line is at the high level after a predetermined time period has passed after the power has been supplied to the controller.
To attain the above object, in a second aspect of the present invention, there is also provided an image forming apparatus to which the above described sheet processing apparatus is attached, the image forming apparatus comprising a second controller that controls the image forming apparatus, wherein the serial communication line performs serial communication from the sheet processing apparatus to the second controller, and wherein the second controller is responsive to supplying of power to the sheet processing apparatus, for determining whether the serial communication line is at a high level, and determines that the sheet processing apparatus is attached when the second serial communication line is at the high level.
Preferably, the second controller determines whether the serial communication line is at the high level after a predetermined time period has passed after the power has been supplied to the sheet processing apparatus.
Also preferably, the image forming apparatus according to the second aspect comprises a power supply control signal line, and wherein the second controller controls supplying of the power to the sheet processing apparatus via the power supply control signal line, and wherein the second controller determines whether the serial communication line is at the high level after instructing the supplying of the power to the sheet processing apparatus via the power supply control signal line.
More preferably, upon returning from a power saving mode, the second controller instructs the supplying of the power to the sheet processing apparatus via the power supply control signal line and then determines whether the serial communication line is at the high level.
Advantageously, the image forming apparatus according to the second aspect further comprises a second serial communication line that performs serial communication from the second controller to the sheet processing apparatus.
The above and other objects, features and advantages of the invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings.